<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script>
         (function (doc, win) {
            var docEl = doc.documentElement,
                resizeEvt = 'orientationchange' in window ? 'orientationchange' : 'resize',
                recalc = function () {
                var clientWidth = docEl.clientWidth;
                if (!clientWidth) return;
                docEl.style.fontSize = 100 * (clientWidth / 375) + 'px';
                };
            if (!doc.addEventListener) return;
            win.addEventListener(resizeEvt, recalc, false);
            doc.addEventListener('DOMContentLoaded', recalc, false);
        })(document, window);
    </script>
    <style>
       html{
          /* 12px */
          /*根字体大小设置为100px,一般不会设置为10px,原因字体最小为12px*/
          /* 
             设计稿 750pt---375px----根100px
             js---375--100
             js---414--110.4(按照比例放大)
             js---320--9X.XX(按照比例缩小)
          */
          font-size:100px;
       }
       body{
          font-size:14px;
       }
       div{
         width:1rem;
         height:1rem;
         font-size:0.5rem;
         background-color: aqua;
       }
    </style>
</head>
<body>
    <div>
        1111
    </div>
</body>
</html>